This invention relates to devices which are adapted for detecting changes in the strength or intensity of magnetic fields and more particularly to such devices which are suited for use in magnetically sensitive fuzes.
Prior art devices for sensing changes in the strength of magnetic fields include flux gate type magnetometers which have been used in anti-tank and anti-vehicle mines. One such magnetometer utilizes a magnetic sensor having a sensing coil wound on a toroidal tape type magnetic core. The sensor is generally designed to operate over the entire flux-field hysterysis loop of its core so that when flux excursions along the hysterysis loop occur in response to large induced voltages due to large external magnetic field disturbances created by external sources, the sensor core saturates with magnetic flux. At the saturation points of the hysterysis loop the flux rate of change with respect to the magnetic field is small, but as the core comes out of saturation such flux rate of change increases in proportion to the slope of the hysterysis loop. If the slope is steep, the increase in flux rate of change and consequently the induced voltage in the sensor coil is high. The induced voltage in the sensor coil is amplified and rectified so as to convert it to a varying DC voltage.
In these flux gate magnetometers, the presence of a vehicle or tank is identified by the particular pattern of the varying DC voltage resulting from disturbance of the earth's magnetic field due to the presence of such vehicle or tank.
Such flux gate magnetometers suffer from long warm-up periods, in the order of 45 seconds, required to permit the magnetometer electronics sufficient time to stabilize, and hence important data could be lost during such warm-up periods. Also such magnetometer utilizes a reference voltage which is dependent upon previously obtained data from external magnetic field changes, and the circuit providing the reference voltage also takes a long time to stabilize. Due to the long warm-up time requirements, if the magnetometer is duty cycled at low on-to-off ratios, the data samples are far apart in time, and short duration data resulting from rapidly passing vehicles in proximity of the magnetometer sensor, will not be detected.
Additionally, the output signal from a flux gate magnetometer is in analog form and it must first be processed into digital format by an A/D converter before a microprocessor or computer can analyze the data. It is also difficult and costly to construct such magnetometer because the sensor coil is wound on a toroidal core upon which a coil with many layers is required, and toroidal cores are difficult to wind.
Further, the circuit of the flux gate magnetometer has a high degree of unreliability designed into it by virtue of having a large number of electronic components.
Importantly, the sensor of the flux gate magnetometers having a toroidal core, suffers from lack of magnetic field directivity, since such sensor responds to field vectors in more than one direction, such as vertical, lateral and radial field vector components.
Another prior art magnetometer used for sensing changes in magnetic fields is of the digital type. This type of magnetometer has a complex magnetic sensor having a core constructed from glass laminate substrates with a magnetic film deposited on each of the laminates. A coil is then wound on the glass laminate core. The electronics associated with the digital type magnetometer consists of an L-C sinusoidal oscillator wherein the L represents the coil of its sensor and the C is a capacitor in parallel with the sensor coil forming a parallel resonant circuit, wherein the LC circuit values determine the sinusoidal oscillation frequency. The output of the oscillator together with a signal from an accurate time base generator are fed into a logic AND gate. The output of the AND gate is fed into counters which count the frequency or repetition rate of the L-C oscillation. The frequency of oscillation is modulated when the inductance of the L-C tank circuit undergoes a change in inductance value due to the sensor being subjected to a change in an external field. This type of magnetometer requires additional magnets which are used to magnetically bias the glass laminate core to a sensitive area of its .phi.-H (magnetic flux versus magnetic field) hysterysis loop.
In view of its glass laminate sensor core construction, the digital magnetometer is unduly complex and costly to fabricate and structurally fragile. Additionally, the required biasing magnets further increase the sensor's complexity as well as it unreliability, and as in the case of the flux gate magnetometer, the digital type of magnetometer also has a large quantity of electronic components which degrade its reliability.
In the digital magnetometer, performance-wise, a change in external magnetic field will not produce a large change in frequency of the L-C oscillator, and consequently this magnetometer is basically a low sensitive device. It has been experimentally determined that when the flux in the coil of the LC oscillator, as measured by a gaussmeter, changed one-half gauss, the frequency of oscillation changed only by one-tenth of a percent.